Lithographic printing is a well known art. In lithography a printing master or plate is employed having a printing surface on which the printing image areas are ink receptive whereas the non-printing background areas are water receptive. During printing, a generally conventional aqueous so-called "fountain solution" is applied to the printing surface of the plate. The fountain solution wets only the water receptive background areas. An oil based ink then is subsequently or simultaneously applied to the printing surface of the plate and is repelled from the background areas, adhering only to the printing image areas. The printing plate is applied directly to paper printing image areas, as known in so-called direct printing or as in offset lithography, the inked printing plate applied to a rubber blanket onto which the image is applied, with the rubber blanket then contacting a paper for transfer of the image thereon.
Lithographic printing plates can be prepared by electrophotographic imaging.
Typically in one electrostatic imaging process a lithographic printing plate is prepared by employing a conventional electrophotographic printing element such as for instance a selenium plate. The plate is charged and exposed to an image pattern. There follows the step of developing such image pattern by attraction thereto of electroscopic toner powder which is ink receptive. There follows transferral to a water receptive plate such as treated paper, grained aluminum and the like. The toner is fused by solvent vapour or heat to form ink receptive printing areas thereon.
A lithographic printing plate can be prepared by developing with ink receptive toner the latent electrostatic image carried by a sheet having a photoconductive zinc oxide coating contained within an insulating resinous binder material. Such coating is generally water repellent. After toning the remaining bare water repellent surface (of the non-imaged areas) is then rendered water receptive by the application of so-called conventional conversion solutions.
Notwithstanding the advantages of the above methods of lithographic printing plate preparation, certain disadvantages are encountered in conventional processes. For instance, in an electrostatic process cited wherein a lithographic plate is prepared by transfer of ink receptive powder deposit onto a water receptive substrate, image resolution is limited by the relatively large particle size of the developing powder and image detail may be lost during the powder image transfer step. Where a binder type electrophotographic plate is employed, the resulting plate has relatively short run life, reducing the number of copies obtainable from such plate to a few hundred to a few thousand copies. Other methods require relatively lengthy preparation procedures and/or long exposure times. In addition, expense of the resulting plates is a factor.
The pending applications Ser. Nos. 632,590 and 656,162 deal with coatings which are applied selectively to the photoconductive layer surface to render the covered areas hydrophilic. A chromic acid compound resulting from application of acidified chromic ion is suggested. While successful, however, some aspects lead to disadvantage as to marketability. These include the anti-environmental impact of chromium compounds and their toxicity which deter their use.
The application of selectively located physically held coatings has been suggested. These also have been successful. Nonetheless, other alternatives that are effective, long lasting, inexpensive, etc. are desired. It is believed that the invention herein provides such alternative with even greater advantage than heretofore available.